Scott J. McCormack

To predict the thermochemistry and thermophysical properties and oxidation mechanisms of titanium diboride (TiB2), we have developed a ReaxFF reactive force field in which the parameters are trained against a set of quantum mechanics data, including the heats of formation and elastic properties for various titanium boride phases, as well as heats of formation of various titanium oxide and boron oxide phases. The developed ReaxFF accurately reproduces the formation energies, relative stability, and elastic properties of these phases. To demonstrate the applicability of our developed ReaxFF force field, we performed ReaxFF-based molecular dynamics simulations to evaluate thermal expansion and melting behavior and validated the results against published experimental data. The thermal expansion simulations reflected the experimentally observed anisotropic behavior, demonstrating greater expansion along the c-axis compared to the a-axis, and showed good agreement in volume expansion measurements. The predicted bulk melting temperature of 3120.87 K is consistent with experimental values (3063–3498 K), while a lower surface melting point of 3049.20 K was observed due to reduced atomic coordination. Moreover, the oxidation behavior of TiB2 was investigated using molecular dynamics simulations. To accelerate the oxidation process within the accessible MD time scales, we employed elevated oxygen concentrations and high-temperature conditions. Additionally, replica exchange molecular dynamics (REMD) simulations were conducted at a temperature of 1970 K, corresponding to experimental oxidation conditions. The simulations revealed that oxidation was initiated with the formation of titanium oxides, consistent with the lower formation energy of TiO2 compared to B2O3. Notably, the titanium oxide formed during the REMD simulations mostly resembled the TiO2 phase in terms of crystallinity. Smaller boron oxide species were also observed through REMD simulations. These results confirm that the developed ReaxFF potential reliably models both the thermochemical and thermophysical behavior of TiB2, as well as complex oxidation mechanisms, making it an accurate and computationally inexpensive tool for simulating high-temperature ceramic materials.

Immiscibility was investigated in Group IV – Group V transition metal diboride pseudobinary systems (TiB2 – NbB2, TiB2 – TaB2, ZrB2 – NbB2, ZrB2 – TaB2, HfB2 – NbB2, and HfB2 – TaB2), revealing classic eutectic lamellar microstructures. Autocorrelation analysis of the microstructure length scales was used to validate the eutectic nature of these microstructures. These results indicate that substantial revisions are required for the pseudobinary phase diagrams of Group IV – Group V transition metal diborides available in the literature and explain several discrepancies between available thermochemical literature and reported microstructure results in these systems.

High temperature crystallographic evolution of graphite determines the fundamental behavior and limitations of this critical material in extreme environments. In this study, the conical nozzle levitator was used in combination with high energy X-ray diffraction to study the crystallographic and atomic structure of graphite up to 2500 °C. The rhombohedral (3R) allotrope of carbon is related to the hexagonal (2H) allotrope of carbon through stacking faults. The annealing of these stacking faults was tracked as the 3R allotrope transformed into the 2H allotrope at elevated temperatures. Highly anisotropic lattice expansion and resulting coefficients of thermal expansion were catalogued and compared to literature values. Atomic displacement parameters were extracted using the Rietveld refinement method, revealing strong covalent bonding in the basal plane of the carbon layered structure that persisted at elevated temperatures and increasingly weak Van der Waals bonding perpendicular to the basal plane between the layers as a function of temperature. The results therein can be used to aid in modeling of directional deformation and shear behavior of graphite at elevated temperatures.

High temperature crystallographic evolution of graphite determines the fundamental behavior and limitations of this critical material in extreme environments. In this study, the conical nozzle levitator was used in combination with high energy X-ray diffraction to study the crystallographic and atomic structure of graphite up to 2500 °C. The rhombohedral (3R) allotrope of carbon is related to the hexagonal (2H) allotrope of carbon through stacking faults. The annealing of these stacking faults was tracked as the 3R allotrope transformed into the 2H allotrope at elevated temperatures. Highly anisotropic lattice expansion and resulting coefficients of thermal expansion were catalogued and compared to literature values. Atomic displacement parameters were extracted using the Rietveld refinement method, revealing strong covalent bonding in the basal plane of the carbon layered structure that persisted at elevated temperatures and increasingly weak Van der Waals bonding perpendicular to the basal plane between the layers as a function of temperature. The results therein can be used to aid in modeling of directional deformation and shear behavior of graphite at elevated temperatures.

The environmental conical nozzle levitator system enables cooling trace experiments above 4000 K. These cooling trace experiments were used to determine the melting points through observation of thermal arrest of group IV and V transition metal diborides (TiB ₂ , T _m = 3313 ± 33 K; ZrB ₂ , T _m = 3404 ± 38 K; HfB ₂ , T _m = 3529 ± 33 K; and TaB ₂ , T _m = 3284 ± 10 K; note: mean ± 2 standard error). Temperature measurements were conducted utilizing dual single‐color pyrometers at wavelengths of 0.9 and 0.65 µm. This system utilizes aerodynamic levitation and dual laser heating to achieve temperatures approaching 4000 K. The samples were synthesized from commercial powders using ceramic gel‐casting to produce high‐density spherical components. The measurements obtained were compared to previously reported melting temperature values and were generally found to be in agreement with most of the published values.